Optical fiber communication systems (“OFCS”) use modulated light to transmit information over optical fibers. Unlike electronic transmissions, optical transmissions are not susceptible to electromagnetic noise and interference, and provide very broad bandwidth. Light emitting devices, such as laser diodes, are used to produce light pulses that are transmitted on the OFCS. Examples of suitable laser diodes include distributed feedback (“DFB”) lasers, Fabry-Perot (“FP”) lasers, and vertical-cavity surface-emitting lasers (“VCSELs”).
It is generally desirable that a light emitting device used in an OFCS produce pulses having a selected light power. Pulses that are too dim might not be reliably received by a photodetector at the opposite end of the optical fiber link, and pulses that are too bright might create an eye-hazard for a human operator. Light output (i.e. pulse light power) from a laser diode is a function of bias current, and increasing or decreasing the bias current increases or decreases the light output. However, different laser diodes will produce different light intensities for the same bias current. In other words, the light produced varies from part to part.
Binning is used to separate diode parts according to their operating characteristics, such as threshold current and slope efficiency. The laser diodes are used in circuits that provide external control for setting the biasing current so that it is suitable for a particular application. This approach requires extra pads to set the desired biasing current, and one-by-one testing to determine which bin each die is sorted to.
Aging and temperature can affect the bias current needed to achieve the desired light power. The control bits used to set the bias current in a particular application might not be sufficient to overcome changes in light power arising from aging or temperature effects. Closed-loop systems have been developed to compensate for changes in laser diode operating characteristics arising from aging and/or changes in temperature.
A closed-loop system is generally a feedback system that detects, evaluates, and compensates for changes in laser diode operating characteristics. This can ensure that a laser diode is able to operate at the desired bias point. Many different methodologies are used in the design and implementation of closed-loop monitoring system.
One closed-loop system computes the slope efficiency of a laser diode by reading the output light power at two different bias current levels of the power versus bias current curve. Both light power readings are taken at a power level above the minimum output light power, which occurs at a bias current above the threshold current. However, this assumes that the slope efficiency remains constant over time and temperature, which it does not.
Another closed-loop system computes the slope efficiency by measuring light power over a range of bias currents, and then sets the target bias current in a step-wise fashion based on the measured threshold current. However, this approach can take a long time, especially if the light power drifts outside the target range. Even if the light power is within the target range, determining the exact value of the threshold current is quite difficult and can take several measurements. Therefore, an improved technique for quickly and accurately determining the bias current for a particular light power from a laser diode is desirable.